Ion pump



E. o. LAWRENCE ETAL 2,726,805

Dec. 13, 1955 ION PUMP 3 Sheets-Sheet 1 Filed Jan. 29, 1953 05. Mm W N@HHH m s H 5. i M \Nw & a E i J E n s L? .1 L Y kw mm B E E c m. i m ggi i Nm fl II I y e a a |||bI, I fl 9 km E 9 Q lllzi. Q t S wv E 9 9 mlE ATTORNEY Dec. 13, 1955 E. o. LAWRENCE ETAL 2,726,805

ION PUMP Filed Jan. 29, 1953 3 Sheets-Sheet 2 ATTORNEY INVENTORS ERNESTO. LAWRENCE JOHN S. FOSTER, JR.

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SUPPLY 3 Sheets-Sheet 3 POWER SUPPLY E. Q. LAWRENCE ET AL POWER SUPPLYFIG.8

Dec. 13, 1955 Filed Jan. 29, 1953 INVENTORS ERNEST O. LAWRENCE JOHN S.FOSTER, JR.

ATTORNEY 0:. o o 0 Q o zozozozoz F I G. 5

FIG. 4

United States Patent 1 2,726,805 ION PUMP Ernest 0. Lawrence, Berkeley,and John S. Foster, Jr.,

Livermore, Calif., assignors to the United States of America asrepresented by the United States Atomic Energy Commission ApplicationJanuary 29, 1953, Serial No. 333,928 13 Claims. (Cl. 23069) The presentinvention relates to an improvement in vacuum pumping and in particularto method and means for electromagnetically pumping large volumes of gasat very low pressures without the use of a pumping fluid.

Conventional vacuum pumps are either mechanical or employ a pumpingfluid to entrain gas for removal, and each of these types of pumps hascertain well-known limitations. The present invention contemplates theprovision of wholly electromagnetic pumping means having no moving partsand requiring no pumping fluid, to there by overcome the limitations ofprior known vacuum pumping means.

It was early recognized that an electron discharge in a closed envelopetended to reduce the pressure therein and numerous attempts have beenmade to take advantage of this phenomenon. Upon this basis was developedan ion gauge, Penning, Patent No. 2,197,079, and various other devices,and this pressure reduction idea was employed by W. v. Meyer, asreported in Z. Physic, 84,531 (1931), for vacuum cleanup in very highvacuum Work with small containers. Following these early disclosuresthere have been developed innumerable devices designed to pump gas andoperating by an electron discharge; however, to date all of thedevelopments have been limited to very small volume gas transfer. Itwould appear from the art that the phenomenon of ionization whenemployed for pumping is a very low order effect and that it could not beemployed to provide large pumping speeds. While such is the clearpurport of the prior art, the present invention shows this not to be thecase when the phenomenon is properly applied and controlled, and thepresent invention provides electromagnetic pumping means having pumpingspeeds comparable to a plurality of conventional vacuum pumps incombination. Not only is the present invention capable of providing veryhigh pumping speeds, but also it is much more economical thanconventional vacuum pumps in many circumstances and is, in fact, suitedfor pumping in certain circumstances where conventional vacuum pumpscannot be employed.

Accordingly, it is an object of the present invention to provide animproved electromagnetic vacuum pump.

It is another object of the present invention to provide an improvedmethod and means of pumping gas by ionizing the gas and moving same asions to a remote point where the gas is neutralized and retained.

It is still another object of the present invention to provide animproved ion pump having a high pumping speed and capable of attaining avery high vacuum.

It is yet another object of the present invention to provide an improvedion pump having means for ionizing gas and transferring same in ionizedform together with means for limiting return gas flow.

It is a further object of the present invention to provide an improvedvacuum pump having a magnetically collimated arc discharge for ionizingand removing gas from a vessel to be evacuated and means limiting gasflow in the pump to a single direction.

Numerous other advantages and possible objects of the invention willbecome apparent to those skilled in the art from the followingdescription of construction and operation of a single preferredembodiment of the invention taken together with the attached drawingswherem: I

ice

Figure 1 is a plan view of the pump attached to a vessel to be evacuatedand partially broken away to show the attachment;

Figure 2 is an elevational view of the pump in longitudinal centersection taken at 2-2 of Figure 1;

Figures 3 and 4 are sectional views of the pump -ends taken at 33 and4-4, respectively, of Figure 2;

Figure 5 is an end section of the pump taken at 5--5 of Figure 1;

Figure 6 is a transverse sectional view taken at 6-6 of Figure 3 andshowing cathode details;

Figure 7 is a sectional view taken at 77 of Figure 3 and showingmounting details of the constriction tube; and

Figure 8 is a schematic representation of the pump including associatedelectrical circuits.

The invention in general as shown in Figure 8 comprises means forestablishing a magnetically collimated oscillating electron dischargebetween negatively charged electrodes and means for admitting gas to thecenter of the discharge whereby same is ionized to produce an arc plasmahaving a voltage gradient between the center and ends thereof. Theionized gas thus travels in the arc plasma to the ends thereof where itis neutralized at the electrodes, and limiter or constriction tubesdisposed about the are adjacent each end electrode force gas diffusingback toward the air inlet to pass through the arc Where it is reionizedand re-delivered to the electrodes. The high electron current in theplasma is a very good ionizing agent so that large volumes of gas areionized thereby, and in addition the arc plasma provides a oneway pathfor ionized particles away from the arc center so that gas is removedfrom the arc center and delivered to the ends thereof to produce apumping action. This pumping action when properly controlled is capableof a pumping speed of many thousands of liters per second and willoperate to produce a vacuum of the order of 10- millimeters of mercury.

Considering now the structure of a preferred embodiment of the inventionas illustrated in the drawings, there is provided a generallycylindrical vacuum envelope 1.1 which is formed of metal and which mayinclude a pair of end sections 12 and a communicating center section 13,with the center section 13 thereof having a greater diameter than theend sections 12 and being joined to the end sections in vacuum-tightconnection. An inlet aperture 14 is provided in the center of vacuumenvelope 11 for communication with a vessel 15 to be evacuated and tothis end center section 13 has one side thereof open and flanged wherebysame may be connected in vacuumtight relation to vessel 15.

There is provided means for establishing a magnetic field longitudinallythrough envelope 11, and this may include a plurality of magnet windings16 disposed endto-end about the end sections 12 of envelope 11 and woundupon flanged cylinders 17 which may be rigidly joined together as byboits through the flanges to form the equivalent of a single, longmultiple-turn solenoid abouteach of the end envelope sections 12. Themagnetic field through the center of envelope 11 and between endsections 12 thereof may be best provided by a large open helix 18 formedof heavy hollow conductor and disposed longitudinally within envelopecenter section 13. Helix 18 may be mounted and supported by rigid hollowelectrical conductors 19 attached to the ends thereof and extendingthrough the top of center section 13 in rigid and insulated relationthereto to suspend helix 18 therein. These conductors 19 areelectrically joined to terminals exterior to the pump proper and theseterminals,

. which are adapted to have suitable energizing means connected theretothrough a plurality of electrical leads adapted to carry a heavycurrent, as shown, are mounted in ins'ulated relation upon thevessel tobe evacuated. A suitable source of cooling water may also be attached toconductors. 1? for providing flow of cooling medium therethrough toprevent overheating thereof from the large currents'necessarily passedtherethrough to establish a strong magnetic field with therelatively fewturns 7 thereof.

' either individually supported or may be rigidly mounted,

as by welded brackets, upon vessel 15, as shown; Under certainconditions of operation, as noted below, it is necessary to provide abacking pump operating in conjunction with the present invention andthere may thus be provided a vacuum manifold 22having an outlet pipeconnected thereto and being adapted for connection to the inlet'of aroughing pump. Vacuum manifold 22 communicates'with envelope 11 throughapertures 23 in one or both ends thereof and valves 24 may be placed inthese connections.

With regard to the internal structure of the pump,

there is provideda pair of cathodes 26 and 27 disposed one at'each endof envelope 11 and interior thereto. At

least one of these cathodes is electron emissive to provide electronsfor the arc discharge within envelope 11, and in the illustratedembodiment the cathode at the right in Figure 2 is electron emissive,the other cathode 26 serving only as an electron reflector. Cathode 27may have any of a variety, of configurations; however, as it is subjectto heavy. ion bombardment, it is desirable for it to be quite sturdilyconstructed. One or more cold cathodes, such as aluminum, may beemployed or a heated cathode may be used, the one illustrated in Figure3 comprising an indirectly heated cathode.

Cathode 27 comprises a closedbox 28 having heat shields on all interiorwalls thereof except the front wall, which is preferably formed of avery hard and. rugged material to with- 7 stand 'ion bombardment. Thefront wall may be formed of an electron emissive material such astungsten ormay, as shown, be formed of a refractory material such ascarbon with a central plug of tungsten or the like; in either instancethe exterior of the front wall comprises in whole or in part an electronemissive surface 29. Electron emission is produced by indirect heatingof emissive surface 29 and there is provided for this purpose afilament'31 disposed within box 23. Filament 31 is energized through apair of rigid hollow electrical conductors 32. V which are placed inelectrical contact with opposite ends of filament 31 by connectors 33.The ends of filament 31 extend through the back wall of 190x28 with atleast one end of filament 31 being outof electrical contact with box 28and the filament ends are held in clamped relation to connectors 33which, in'turn, are apertured to each receive one of the electricalconductors 32. An insulator is provided between connectors 33 and apassageway is formed between the apertures in connectors 33 through theinsulator to provide communication between hollow conductors 32 andthereby provide a path for cooling water from a source thereof attachedto conductors 32 exterior to envelope 1 The above-noted cathodestructure 27 may be suspended upon conductors 32, as shown, andconductors 32 may, in turn, be mounted in insulating relation throughthe end plate 34 of a cylinder 36 which extends within envelope 11 aboutcathode structure 27 and is apertured to communicate with aperture 23 inenvelope 11. End plate 34 is mounted, as by bolts, to the end of themagnet winding structure and attached envelope 11 and is insulatedtherefrom as by a cylindrical insulator 37 which also serves as a partof the vacuum vessel by closing the gap between. the end of envelope 11and an inwardly projecting extension of end plate 34. The entire cathodestructure including cylinder 36 and conductors 32 are insulated from theenvelope 11, and the magnet winding cylinders 17 and conductors 32 areadapted for connection to a source of cooling water, not shown, and

to suitable power supplies, as noted below. Inasmuch as a large amountof heat is produced by ion bombardment within envelope 11 there may beprovided cooling tubes about same and about cathode'cylinder 36; whichtubes are also connected to a source of cooling water,'not' shown. A.gas inlet tube 35 extends through envelope 11 in the vicinity of cathode27 and may be connected ex ternally of envelope 11 to a source of gas(not shown).

At the opposite end of envelope 11 there is disposed the other cathode26 and while it may be constructed the same as cathode 27 it need onlyfunction as an electron reflector and thus need not be electronemissive, cathode 27 providing sufiicientelectrons to establish andmaintain the desired discharge. As illustrated in Figure 4,

reflector cathode 26 includes a block 38 of metal which.

may have a removable face plate and which has a pair of rigid hollowelectrical conductors 39 secured to the back thereof. Conductors 39extend exterior to envelope 11 and they may engage the end structure ofthe magnet winding cylinders 17 in much the same manner as do conductors32 of cathode 27. A cylinder 41 may be dis posed about cathode '26interior to envelope 11 and there may be provided cooling tubesthereabout, as shown. Cathode 26, conductors 39, and cylinder 41 aremounted in insulated relation to envelope liby a cylindrical insulator42 similar in construction and mounting to insulai tor 37 of cathodestructure'27. Cathode conductors 39 are adapted for connection to asource of cooling water exterior to envelope 11 which circulates waterthrough conductors 39'and a communicating passage in block38,

and conductors 39 are also adapted for connection to suitablepowersupplies, as noted below. 7

Interior toenvelope 11 there are provided a pair of like constrictiontubes 43 and 44 disposed adjacent oathodes 26 and 27 respectively.Constrictiontubes 43 and-44 are disposed concentric with envelope 11 andeach has a diameter substantially equal to that of the'arc dischargethat is supported within envelope 11. Tubes 43 and 44 have outwardlyextending'fianges 46 and 47, respectively, 7

at adjacent ends thereof and these flanges engage the inner surface ofenvelope '11, Preferably in slidable relation.

Constriction tubes .43 and 44-. may be secured in position by anysuitable means; however, it is desirable to mount tubes 43 and (Minremovable relation to envelope 13 and to this end'there may be providedsplit rings 48 disposed about an end of each tube. 'As shown in Figure7, ring 48 has a tapered split therein and awedge plug 49'is disposedtherein with a bolt 51 extending through a flangeabout the end of tube44 and threadably engaging wedge plug 49. Tightening bolt 51 urges wedgeplug 49 furmounting means for both'constriction tubes 43. and 44' supply53 which may have the negative terminal thereof 7 connected to cathodes26 and 27, as at leads 39 and 32 thereof respectively, and the positiveterminalgrounded.

' With the above connection of power supply 53, the envelope 1 1 isgrounded as shown, and while this'circuitry is advantageous in that theenvelope insulation problem is minimized, it willbe appreciated that itis only nece s sary' to provide a voltage difference between envelope 11and cathodes 26 and 27 with neither requiring grounding. Magnet currentmay be provided by a single power supply or may, as shown, be providedby power supplies 56, 57, and 58 with power supplies 56 and 58 eachconnected across one of the magnet end sections including seriallyconnected windings 16' and power supply 57 connected across helix 18.Inasmuch as helix 18 has much fewer turns than magnet windings 17, it isnecessary to provide a much higher magnet current through helix 18 inorder that a substantially uniform magnetic field will be producedthroughout the length of envelope 11. In addition to the above-notedelements and connections there may be further provided suitable controland interlock means to conveniently operate the system either manuallyor automatically.

Considering now the operation of the illustrated embodiment of theinvention and referring generally to Figure 8, the pump is firstconnected to a vessel 15 to be evacuated with communication beingprovided therebetween at aperture 14 in the center section 13 of pumpenvelope 11. Energization of magnet windings 16 by power supplies 56 and58 and energization of helix 18 by high current power supply 57 producesa strong magnetic field having lines of force passing axially throughenvelope 11. Energization of cathode 27 is accomplished by connectingpower supply 52 across filament 31 to cause a heavy current flowtherethrough. The temperature of filament 31 is raised and heat isradiated therefrom to raise the temperature of the electron emissivesurface 29 of cathode 27 so that electrons are emitted therefrom.Connection of power supply 53 between envelope 11 and cathodes 26 and 27places envelope 11 at a relatively positive potential with respect tocathodes 26 and 27 so that envelope 11 efi'ectively comprises an anodeand electrons are attracted from cathode 27. The axial magnetic fieldthrough anode envelope 11 collimates the electron discharge andconstrains the electrons to pass through envelope 11, actually in tighthelixes about magnetic lines of force, until they approach cathode 26,at which point they are repelled by the relatively negative chargethereon. The electron discharge established within envelope 11 is thusoscillatory in that electrons oscillate through anode envelope 11between cathodes 26 and 27 under the constraining influence of themagnetic field and the repelling and attracting forces of the cathodesand anode envelope, respectively.

Admission of air or gas into envelope 11, as through aperture 14,results in ionization thereof by the electron discharge and because ofthe elongated electron path the probability of ionization is very high.The electron discharge thus becomes a high intensity arc discharge withthe arc plasma extending as a cylinder through envelope 11 into closeproximity with cathodes 26 and 27. As is the characteristic of arcs, thearc discharge assumes substantially the potential of the most positiveelectrode producing same so that it has approximately the potential ofanode envelop 11. Almost the entire potential drop occurs in a veryshort distance between the arc ends and cathodes 26 and 27 and for atypical installation this may be about 500 volts, i. e., cathodes 26 and27 may be maintained some 500 volts below ground potential.

With regard to the arc discharge established, it is first noted that theelectrons thereof have an elongated path and pass back and forththerethrough losing energy by excitation and ionization until theyeventually fall out of the arc and are collected by the anode walls. Thepositive ions of the arc, being of much greater weight than theelectrons, have a much lower velocity and a short mean free path so thatthey develop a positive space charge in the arc column. This charge isgreatest at the center of the are so that the positively charged ionsmove away from the arc center toward the ends thereof, being laterallyconfined within the arc column by the magnetic field. Eonization ofincoming gas is accomplished by electrons and ions within the arc andsubstantially all gas molecules entering anode envelope 11 throughaperture 14 are ionized very close to their entrance point. The use ofhelix 18 provides means for establishing substantially the same magneticfield through center section 13 as is provided in end sections 12 bymagnet windings 16 and at the same time presents an open structurethrough which incoming gas may readily enter the arc.

The gas pressure in anode envelope 11, and thus communicating vessel 15is thus reduced by the above-noted process wherein gas molecules areionized and transported away from the arc center. As the ions reach theends of the are they are accelerated across the short gap between thearc ends and the cathode to impinge upon the cathode. The ions strikingthe cathode may either combine with the cathode material and stay thereor may pick up an electron to become a neutral particle 01' gasmolecule. The ion bombardment of the cathode surface causes somesputtering of the electron emissive surface thereof and this isdeposited upon adjacent surfaces to produce a gettering action wherebysome of the ions neutralized at the cathode are trapped on thesesurfaces. Of the remaining neutral particles some at least will difiuseaway from the cathode back along envelope 11 toward the center thereofand under normal conditions would tend to raise the pressure at theenvelope center. This gas flow is prevented in the present invention byconstriction tubes 43 and 44 which have almost the same diameter as thearc column. Gas flowing away from cathodes 26 and 27 toward the airinlet 14 must pass through constriction tubes 43 or 44 and are thusreionized by the arc discharge therein. No other opening is availablefor gas to flow back toward air inlet 14 and thus no gas can reach airinlet 14 from cathodes 26 and 27 without passing. through the arcwherein it is reionized and redelivered to the adjacent cathode.

With regard to the backing pump, it will be appreciated that with a basevacuum being maintained in vacuum manifold 22 some of the ionsneutralized at the cathodes will pass into the vacuum manifold and bepumped away. EX- perimentation has shown that unless an inert gas isbeing pumped by the improved ion pump there need not be employed abacking pump and exhaust valves 24 may be closed, the chemical processat the cathodes reaching equilibrium with the pumping speed so thatsubstantially all incoming gas eventually combines with the cathodematerial.

Considering the relative magnitude of the abovementioned phenomenawithin the pump, it is noted that the pumping speed of the pump isdependent upon a variety of factors and may be defined as:

Q AP wherein:

S is pumping speed AL is change in gas volume inlet AP is pressurechange produced by AL S a VaA wherein Va=Arithmetic average molecularvelocity K=A constant depending upon the overall eflicient of the are asa sink for molecules.

The value of K has been found to be about 0.5-0.3 in

a typical .pump designed in accordance with the present invention andthe pumping speed may be about 75 liters/sec/square inch. A totalpumping speed of some 6000 liters/sec. may thus be readily achieved. Thebase able as in the illustrated embodiment of the invention to pressureof the pump is the summation of a number of eflfects, as for example,outgassing of the internal pump elements; however, after a short periodof operation this is reduced to such a low order as to be negligible. Acer tain portion of the base pressure is generated by ions falling outof; the arc and striking the anode walls wherein the magnetic field theyhave a random motion and the pump envelope is made long enough that inthe distance between constriction tube ends 46 and 47 and pump inlet 14there is a high probability that the gas molecules will fall into thearc and become reionized. It is also possible that neutral particles maybe formed'within the are as a result of electron-ion attachment;however, this is normally considered a rare event in gaseous discharges,but does play a significant part in the present device.

. Minimization of back pressure is obtained by the above notedelongation of the pump to limit back diffusion of gas molecules formedat the pump ends and by constricting the anode diameter adjacent thecathodes to preclude back flow of gas molecules from the cathodes. Also,variation of the hot cathode temperature is effective to control orlimit radiofrequency plasma oscillations which may be set up and whichtend to increase ion drain across the magnetic field. Further, the arcvoltage employed determines in part the relative magnitudes ofrecombination within the arc the ionization thereof, and theexcitation.' The magnetrc field intensity along the anode envelope isalso'of importance and .generally optimum results are obtained with auniform field strength.

V In summary it may be noted that the present improved ion pump providesa high intensity magnetically collimated arc for ionization andtransportation of gas to be pumped. A relatively large are surface isexposed to the inlet aperture at the arc center for maximum pumpingspeed and the anode envelope diameter is constricted immediatelyadjacent the end cathodes which in combination with the cylindricalanode and axial magnetic field establish a refluxing arc discharge forefficient ionization. The relatively low ion mobility compared with theelectron mobility in the arc'produces an electric field gradient alongthe are which urges ions away from the are center toward the cathodes.Ions reaching the cathodes are'there neutralized and neutral gasmolecules difiusing back toward the pump inlet must pass through theanode constriction so that they are reionized by the are whichsubstantially fills the constriction.

A pressure differential is maintained between the pump inlet 14 and thecathodes, and in a typical pump the pressure in center section 13 may beabout to Gil ficient pressure at one cathode and the pump inlet pres- 7sure falls much lower than the cathode pressure. The

anode constrictions prevent reverse gas flow of gas let in at thecathode and thus the arc is in effect maintained to effectuate pumpingin a pressure below that at which the arc extiuguishes.

It will be appreciated that numerous modifications and variations of theinvention may be made within the spirit and scope thereof as for examplethe vacuum manifold may be attached to either or both ends of the anodeenvelope. vention shall be limited by the present disclosure of 'asingle preferred embodiment but rather attention is invited to thefollowing claims for a precise definition of the invention.

What is claimed is:

1. An ion pump comprising an elonga'ted'envelope, inlet meanscommunicating with the interior of said en-' velope adjacent thelongitudinal center thereof and adapted for connection we vessel to beevacuated, means establishing an electron discharge longitudinally ofsaid envelope for ionizing gas entering same through said inlet meansand thereby producing an arc discharge 7 within said envelope, meansremoving ions from the ends of said are discharge and neutralizing same,and means directing return gas flow from the ends of said are dischargethrough the discharge wherein same is .reionized 10- mm. of mercurywhile the pressure at the cathodes is about 5X10- mm. of mercury. Acertain minimum amount of gas molecules is required to maintain the arewhich otherwise extinguishes and 'in the present invention this gas maybe provided through gas leak tube 35. An external gas source (not shown)is connected to tube SS which then feeds gas into one end of the areadjacent cathode 27. 'A particular advantage of the present invention ishere apparent for the gas pressure need only exceed the minimum at thearc end to maintain the arc and none of the gas admitted through tube 35passes along envelope 11 to raise the pressure at the pump inlet. Theare ismaintained by holding a suf and redelivered to the arc dischargeends, thereby'providing a pumping action from the center tothe ends ofsaid envelope.

2. An ion pump comprising anelongated envelope, a pair of separatedelectrodes disposed within said envelope with at least one of same beingelectron emissive,

means establishing a magnetic field through said envelope directedbetween said electrodes, power supply means connected between saidenvelope and electrodes for main taining said electrodes electricallynegative withrespect to said envelope whereby an oscillating electrondischarge is maintained Within said envelope between said electrodes,inlet means communicating with the interior of said envelope midwaybetween said electrodes and adapted for connection to a vessel to beevacuated whereby gas therefrom is ionized by said electrondischarge toproduce an arc plasma within said envelope, ionization at the middle ofsaid arc plasma producing a potential gradient therein whereby ions inthe plasma travel to said'electrodes and are there neutralized, andconstriction means about said arc plasma adjacent said electrodes fordirecting gas dilfusing from said electrodes toward said inlet meansthrough a substantial length of said are plasma whereby said gasis'reionized and returned to the electrodes so that the arc plasmaremoves gas from the envelope center. a

3. An improved ion pump comprising an envelope, means establishing anoscillating electron discharge within said envelope, inlet meanscommunicating with the interior of said envelope adjacent the middle ofsaid discharge and adapted for connection to a vessel to be evacuatedwhereby gas enters said envelope and is ionized by said discharge, saiddischarge thereby becoming an arc discharge including a plasma in'whichionized gas molecules migrate to the ends thereof, means magneticallycollimating said are discharge, and limiting means. disposed at the endsof said are plasma for substantially ing ions reaching the end' of saidare and constrictions in said envelope about said arc adjacent eachendthereof whereby neutralized ions diifusing from the arc ends towardthe arc center are constrained to travel through the .arc whereby sameare reionized and directed away from the are center.

.5. An ion pump comprising an envelope having an It is thus not intendedthat the in-.

aperture at the middle thereof for communication with a vessel to beevacuated, means including a pair of electrodes disposed one at each endof said envelope for establishing an oscillating electron dischargetherein whereby gas entering said envelope through the aperture thereinis ionized to produce an arc discharge between said electrodes, andrestriction means between each of said electrodes and the center of saidenvelope, said means each having a passage therethrough of substantiallythe same diameter as the arc discharge whereby neutral particles cantravel from said electrodes toward the center of said envelope onlythrough said are discharge.

6. An improved ion pump comprising an envelope having a pair of gasoutlets disposed one at each of the ends thereof and a gas inlet at thecenter thereof, means establishing an ionizing arc discharge throughsaid envelope between said gas outlets and past said gas inlet with thecenter of said discharge being electrically positive with respect to theends thereof whereby gas entering said inlet is ionized and travels tothe ends of said are whereat same is neutralized, and a pair of envelopeconstrictions disposed with one adjacent each end of said envelope aboutsaid are and each having a passage therethrough of substantially thesame diameter as said arc discharge whereby gas flow from said outletstoward said inlet is constrained to the are discharge area wherein suchgas is reionized and thereby prevented from reaching said gas inlet.

7. An improved vacuum pump comprising a cylindrical envelope having agas inlet aperture at the center thereof, means including a pair ofelectrically negative electrodes disposed one at each of the oppositeends of said envelope for establishing an ionizing electrical dischargein the form of an are through said envelope past said inlet aperture andionizing gas entering therethrough, said are being electrically negativeat the ends with respect to the center thereof whereby ionized gastravels to the ends of said are and is there neutralized at saidelectrodes, and a pair of members disposed within said envelope oneadjacent each of said electrodes about said are and each havingelongated passages therethrough of substantially the diameter of saidare whereby gas at said electrodes can only reach the center of saidenvelope through said passages and is thus reionized therein by said arcand is returned to said electrodes.

8. An improved ion pump comprising an elongated cylindrical anode havingan inlet aperture at the center thereof, a pair of cathodes disposed oneat each of the opposite ends of said anode within same for establishingan ionizing discharge through said anode, means establishing a magneticfield having lines of force longitudinally through said anode forcollimating the discharge therethrough, said discharge ionizing incominggas to become an arc discharge having a voltage gradient urging ionstoward the arc ends whereby ions travel to the arc ends and strike saidcathodes, and a pair of constrictions within said anode cylinderadjacent said cathodes toward the center of said anode cylindertherefrom for limiting gas flow from the cathodes toward the inletaperture to the arc discharge wherein the gas is reionized andredelivered to the cathodes.

9. An improved ion pump comprising an elongated anode cylinder, meansestablishing a magnetic field having lines of force through saidcylinder, a pair of cathodes disposed one at each end of said cylinder,a power supply maintaining said cathodes at a negative potential withrespect to said anode for producing an oscillating electron dischargethrough said anode cylinder collimated by said magnetic field, an inletaperture in said anode cylinder spaced equidistant between the cathodestherein and adapted for connection to a vessel to be evacuated wherebygas enters therethrough and is ionized by said electron discharge toproduce an are discharge therefrom, said ionized gas traveling to saidcathodes where it is neutralized, and constriction tubes disposed withinsaid anode cylinder on the inlet side of said cathodes and adjacentsame, said constriction tubes having a diameter substantially equal tothat of the collimated arc discharge whereby back flow of gas from saidcathodes toward said inlet is constrained to pass through a substantialarc length wherein the gas is ionized and returned to the cathodes.

10. An improved ion pump comprising a cylindrical envelope having anexpanded center section with a large inlet aperture therein adapted forconnection to a vessel to be evacuated, a large open helicalmagnetwinding disposed axially of said envelope internally of the centersection thereof, a plurality of magnet windings disposed about saidenvelope exterior thereto, power supply means connected to said internaland external magnet windings for energizing same to produce a strongmagnetic field axially through said envelope, means including a pair ofcathodes equally spaced on opposite sides or" said inlet aperture forestablishing an oscillating electron discharge through said envelopewhereby gas entering same through said inlet aperture is ionized toproduce a magnetically collirnated arc discharge through said envelopewith the ions thereof producing a space charge establishing a voltagegradient between the center and ends of said arc discharge, ions formedby said arc being delivered to the ends thereof by said voltage gradientand said ions being neutralized at said cathodes, and means directingreturn gas flow from said cathodes through a substantial portion of saidarc whereby said gas is reionized and redelivered to said cathodes.

11. An improved ion pump comprising an envelope, means establishing anelongated magnetically collimated arc discharge through said envelope, apump inlet connected to said envelope at the center of said aredischarge whereby gas entering through said inlet is ionized by said areto form potential gradient between the center and ends of said arethereby urging ions toward the ends of said arc, means disposed adjacentthe ends of said arc for neutralizing ions reaching same, and meansconstricting said envelope to the arc discharge area adjacent the arcends whereby gas diffusing from said ion neutralizing means toward saidpump inlet is directed through said arc and is reionized and redeliveredto said neutralizing means.

12. An improved ion pump comprising an elongated cylindrical envelopehaving a large inlet aperture at the longitudinal center thereof and asmaller outlet aperture at one end thereof, means including an openhelical winding at the center of said envelope for establishing a strongmagnetic field axially through said envelope, and a pair of electrodesdisposed one at each end of said envelope for establishing an electrondischarge therethrough collimated by said magnetic field to a diametermuch less than that of said envelope, said discharge ionizing gasentering through said inlet aperture to become an arc discharge having apotential gradient urging ions to the ends thereof where same strikesaid electrodes and are neutralized, said envelope having a constrictionreducing the diameter thereof substantially to the diameter of said aredischarge immediately adjacent each of said electrodes struck by ionsfrom said arc for limiting the return path for neutralized gas ions tothat of the arc wherein they are reionized and returned to theelectrode.

13. An improved ion pump as claimed in claim 12 further defined by gasinlet means adjacent one of said electrodes on the opposite side of saidenvelope constriction from the envelope inlet aperture for admitting asmall amount of gas to the electrode whereby said arc is maintained atvery low pressures at the envelope center.

References Cited in the file of this patent UNITED STATES PATENTS

